Traveling wave tube phase shifter



April 23, 1.95.8 H. SCHARFMAN 3,379,990

TRAVELING WAVE TUBE PHASE SHIFTER Filed March 9, 1964 BEFORERECTIFICATIQN f /N VEA/7' 0l? HOWARD SCHARF/VAN AGE/VT United StatesPatent 0 3,379,990 TRAVELING WAVE TUBE PHASE SHIFTER Howard Scharfman,Lexington, Mass., assignor to Raytheon Company, Lexington, Mass., acorporation of Delaware Filed Mar. 9, 1964, Ser. No. 350,435 4 Claims.(Cl. S30- 43) ABSTRACT 0F THE DISCLGSURE The invention pertains to atraveling wave interaction device having a plurality of individuallyenergized collector electrodes extending along the interaction path.Precise phase control of propagating electromagnetic signals is realizedby means of the control of the electrical length of the slow wavepropagating structure before the electron beam is completely collected.The energizing means for the individual collector electrodes maycomprise a system of preprogrammed digitalized voltages from a computer.Numerous embodiments including circumferentially and longitudinallystaggered electrodes adjacent to the slow wave propagating structure aswell as split electron beam devices are described.

This invention relates to electronic devices and in particular to atraveling wave type device for providing controlled phase shift of aninput signal.

Basically, a traveling wave tube comprises a wave propagating structure,such as a helix, and an electron beam traveling near the structure witha velocity very near the velocity of propagation of a wave on the wavestructure. An electromagnetic signal is supplied to one end of the helixin the form of a traveling wave having a strong component of electricfield parallel to the direction of electron beam motion. By means ofcontinuous interaction between the electric field on the electromagneticwave and the electron beam, amplification is obtained. The amplifiedelectromagnetic signal is removed from the end of the helix furthestremoved from the source of the electron beam.

It is frequently desirable, for example, in phased array applications tocontrol or alter the relative'iphase shift of signals applied to thehelix. It has beenfound that the degree of phase shift through the wavepropagating structure is a function of the reactive loading of theelectron beam and hence the beam current. Beam current may be controlledby introducing a control element at or near the source of electrons orthrough control of the beam collector voltage. However, phase shiftversus either beam current or control element voltage characteristicshave considerable nonlinearity over the useful dynamic ranges andfrequency bandwidths contemplated. Thus, in precision traveling wavephase shifters based on beam current control, it is necessary to employan external feedback system at increased cost and complexity therebyrendering the use of such devices less attractive commercially.

Accordingly, it is an object of the present invention to provide atraveling wave tube phase shifting device with inherent precision phasecontrol without the aid of an external feedback loop.

It is a further object of the invention to provide a phase shifterhaving negligible hysteresis and excellent resettability.

Still another object of the invention is to provide a precision phaseshifter having characteristics capable of being duplicated from unit tounit in production.

These and other objects of the invention are provided ICC adapted tosupport an electromagnetic signal; a source of electrons, such as anelectron gun, for transmitting a beam of electrons in proximity to saidwave propagating structure; and a plurality of collecting electrodesdisposed adjacent the length of said wave propagating structure andadapted to be energized by an appropriate voltage at predeterminedelectrodes to permit collection of substantially all of the electrons onthe beam at predetermined collector electrodes. In this manner, thedegree of phase shift of the input electromagnetic signal becomesproportional to the length of wave propagating structure traversed bythe beam before being completely collected. Since this length can bepredetermined with great accuracy, the desired degree of precisionwithout feedback becomes a reality.

Other objects, features and advantages of the present invention will bebetter understood by the following description, taken together with theaccompany drawings in which:

FIG. 1 is a schematic view of a traveling wave tube em-bodying the phaseshift collecting elements of the invention;

FIG. 2 is a graph plotting the R.F. voltage versus the collectingelement terminals for a predetermined frequency signal F0;

FIG. 3 is a schematic view of a portion of the traveling wave tube ofFIG. 1 modified to illustrate the staggered arrangement of collectorelectrodes; and

FIG. 4 illustrates a split beam embodiment of the invention.

Referring more particularly to FIG. 1, there is shown a traveling wavetube 11 having a helical slow wave structure 12 aligned approximatelycoaxially within and supported by, for example, a glass envelope 14, Atone end of the tube an electron gun 16 is disposed. The gun is adaptedto project a stream of electrons 17 along the longitudinal axis of thehelix 12 to a collector electrode 18. A magnet, not shown, is normallyprovided coaxial to the tube to focus the beam. Input means 2) andoutput means 22 are provided for coupling electromagnetic energy ontoand oif from, respectively, the slow wave structure. The velocity of theelectron stream and the ratio of the circumference to the pitch ofthe'helix are such as to permit a cumulative electromagnetic interactionbetween the electron stream or beam 17 and the electromagnetic energy onthe helix whereby the electromagnetic energy is amplified. Voltage fromsource V2, impressed across the cathode 19 of electron gun 16 andcollector electrode 18, establishes a fixed beam current therebetween. Acontrol anode 24, in the electron gun 16, controls the magnitude of thebeam current and thereby the gain or amplification of the tube. Thevoltage on the control anode 24 is supplied frorn variable voltagesource V1. It has been found that variation of the beam current not onlychanges the gain of the tube bu-t likewise varies the reactive load ofthe wave propagating structure and hence the degree of phase shift ofthe signal through the structure. Furthermore, the phase shift versusactuating voltage characteristics of beam current phase Shifters isinherently nonlinear without external feedback loops. In order tocontrol the degree of phase shift through the structure in a mannerwhich is relatively independent of beam current and which is capable ofa high degree of precision and linearity, there has been provided, inaccordance with the invention, a plurality of collecting electrodesdesignated T1Tn, wherein n is any number greater than 1 within thelimits imposed by the longitudinal length of the electrode surface, thelength of the helix and the spacing required between electrodes. Theseelectrodes Tl-Tn are spaced apart from and ad- 3 jacent the longitudinalaxis of the helix 12. The individual collecting electrodes Tl-TJn areappropriately energized with predetermined voltage-s iny accordance withthe degree of phase shift desired so that substantially `all of theelectrons on the beam are collected at specified collecting electrodesthereby terminating the interaction at either one specified electrode orat certain electrodes and hence precisely establishing the degree ofphase shift introduced on the signal propagated through the tube. Thephase shift is thus made proportional to the length of slow wavestructure passed by the beam before being substantially completelycollected. For a given tube length and collecting surface the precisionof the phase shifter is limited only by the closeness of spacing betweenthe collecting electrodes. It is, therefore, within the contemplation ofthis invention to locate collecting electrodes circumferen- -tially andlongitudinally staggered about the helix to increase the permissiblespacing tolerance between collector electrodes.

FIG. 3 shows a portion of the traveling wave tube of FIG. l in which thecollecting electrodes are oppositely staggered. Electrodes T2, T3 and T4are disposed in a column in the same locations as the correspondingelectrodes in FIG. l, whereas electrodes T2A and TSA in a second columnare oppositely disposed and longitudinally staggered from electrodes T2,T3 and T4. Several such columns may be provided about the circumferenceof the tube as desired.

It is also within the contemplation of the invention that the electronbeam 17 may be directed so as to pass outside the helix instead ofcentrally through the helix as shown in FIG. 1. It is furthercontemplated that with the beam either inside or outside the helix asplit beam technique may be employed to enable continued amplificationby interaction with one beam and a controllable phase shift from theremaining beam. This embodiment of the invention is shown in FIG. 4. Itis noted that corresponding parts in FIG. 1 have been simil-arlynumbered in FIG. 4. It may thus be seen that the apparatus of FIG. 4 isidentical to that of FIG. 1 with the exception of the control anode 46and the split beams 40 and 42. In a split beam device, the electronsfrom cathode 19 are split into two beams 40 and 42 upon passage throughtwo slits in anode structure 46. The two beams pass by helix 12 andinteract with the signal on the helix in substantially the same manneras the single beam described in connection with FIG. l. One or more ofthe beams may then be collected at, for example, collecting electrodeT5, thus effectively terminating the degree of phase shift introduced bythe beam collected on the signal prop-agating through helix 12. One ormore of the split beams may be permitted to continue along the helixinteracting with the signal and amplifying the phase shifted signal onthe helix. Accordingly, it becomes apparent that with the split beamtechnique simultaneous phase shift and amplification may be achieved oramplification before phase shift or phase shift before amplification asmay be desired by appropriately programming the collecting electrodes.

Many systems may be devised for energizing the electrodes such as by acomputer having preprogrammed, digitalized voltages. Accordingly, the4following system described in connection with FIG. 1 is meant to beexemplary thereof and should in no way be construed as a limitation onthe scope of the invention. Referring back to FIG. l taken in connectionwith FIG. 2, there is shown a suitable system for energizing thecollecting electrodes 'T1-Tn. A waveguide cavity 32 or short circuitedsection is suitably connected as by a coaxial wire to a source ofvariable frequency signal 34. Coupling probes or loops areintroducedinto the cavity and are separately coupled to rectifying diodes 60-69which are, in turn, respectively coupled to collecting electrodes Tl-Tn.For a given frequency of input signal F0, the distribution of R.F.voltage picked up by the loops or probes is as shown in FIG. 2, curve A.High frequency rectifiers 60-69 hold off the voltage at each probe sothat the D.C. voltage distribution at the collecting electrodes 'T1-Tnis as shown in FIG. 2, curve B. The maximum of this voltage distributionis at T4 and this will be the point at which most or all of the beam 17is collected. The maximum of the voltage distribution can be moved bychanging the frequency output of source 34, since the rate of change ofposition of the maximum voltage with respect to frequency change is afunction of the waveguide dispersion which, in turn, is related to thefrequency input.

It should be noted that preprogrammed digitalized voltages from acomputer may be applied to yappropriate collector electrodes in place ofthe phase control apparatus 44 depicted in FIG. 1. Furthermore, a largeror fewer number of steps of incremental phase shift may be obtained byapplying voltages of the same or different magnitudes to a number ofcollectors at once. It is also noted that other suitably dimensionedwave propagating structures may be used in place of the helix shown inFIGS. l, 3 and 4, such as, for example, interdigital lines, ladder linesand vane type lines. Accordingly, it is desired that this invention notbe limited except as defined by the appended claims.

What is claimed:

1. In combination:

means for supporting an electromagnetic signal;

means for transmitting a Ibeam of electrons in proximity to said wavepropagating structure;

a plurality of columns of col-lecting electrodes -adjacent thelongitudinal axis of said wave propagating structure, adjacent columnsbeing longitudinally staggered from each other;

and means for varying the phase of signals propagated through said wavepropagating structure comprising a source of radio frequency signalscoupled to a tuned cavity circuit having means for coupling rectifiedsignals from said tuned cavity circuit to said collecting electrodes.

2. A traveling wave tube phase shifter and amplifier comprising:

a slow wave propagating structure adapted to support an electromagneticsignal;

a source of electrons;

means for amplifying said electromagnetic signal by compelling a beam ofsaid electrons to traverse the longitudinal axis of said slow wavepropagating stnucture in close proximity thereto and at a velocity suchthat interaction occurs between the beam and a signal on said structure;

means for producing a magnetic field extending parallel to thelongitudinal axis of said slow wave propagating structure;

input means coupling said electromagnetic signal to an end of said wavepropagating structure nearest said source of electrons;

output means for removing said electromagnetic signal from said wavepropagating structure;

and phase shifter means for phase shifting said electromagnetic signaldisposed intermediate said input and output means and adjacent thelongitudinal axis of said Wave propagating structure including afplura-lity of spaced collecting electrodes for collecting substantiallyall the electrons compelled from said source at predetermined distancesalong the length of said structure with said distances determined by`the application of preselected individualized D.C. voltagedistributions to each of said collecting electrodes.

3. The apparatus of claim 2 in which the collecting said structure an-dadjacent columns are staggered longitudinally.

4. In combination: a wave propagating structure adapted to support anelectromagnetic signal; a source of electrons; amplifier meanscompelling at least a pair of beams of said electrons to traverse thelongitudinal axis of said Wave propagating structure in close proximitythereto and at a velocity such that interaction occurs between the beamsand a signal on said structure; input means for coupling saidelectromagnetic signal to an end of said wave propagating structurenearest said source of electrons;

output means for collecting all the electrons in at least one of saidbeams at predetermined distances along the length of said structure.

References Cited UNITED STATES PATENTS output means for removing saidelectromagnetic sig- 15 ROY LAKE, Primary Examiner.

nal from said wave propagating structure; and phase shifter meansintermediate said input and NATHAN KAUFMAN, Examinez'.

